Secretary of State Rex Tillerson said the call -- which he sat in on -- was "very productive" with "a lot of detailed exchanges."

"We'll see where we go from here."

No details were given about the possible safe zones, which have long been discussed, but faltered as Bashar al-Assad's regime and assorted groups of rebels, Kurds, Iranian-backed militias, Al-Qaeda and Islamic State fighters have kept-up a bloody war that has killed an estimated half million people.

The Kremlin, meanwhile, said "the emphasis was put on the potential for coordination of actions by the United States and Russia in the fight against terrorism."

The State Department earlier announced that it would send a junior minister to Russian-backed peace talks in Astana later this week.